Staphylococcus aureus infections. Testing the virulence determination, Index of Infections Potential (IIP). The work group includes Dr. David Henderson, members of his Epidemiology Section, Dene Zierdt and myself. In this extended study, true infections will be rigorously determined. For the published work, significant S. aureus isolates were designated from site of isolation, as wound, abscess, blood, and other sterile fluids. The present study will more validly categorize isolates. The number of isolates for a phage pattern will further determine whether an IIP value can be assigned, according to statistical criteria. Application of the IIP to a few phage patterns, for example 92/94/96/292/D- 11, showed a decline in IIP over a period of years, after entry to our hospital wards. This goes along with the understanding that extraordinarily virulent S. aureus strains on entry, lose that quality over a period of years. Progress: We seem to be at a point where we can try Index of Infectious Potential (IIP) on our ORSA and HBLPSA strains. It took a long service to work up the clinical criteria for true infectious strains, and to translate these on standard forms, with computer entry of the data. The best bets for IIP determination are the ORSA and HBLPSA. Other possibilities are 3B/3C/55/71 and nontypable, with the exodus of ORSA strains, and limited numbers of HBLPSA, the Clinical Center presently has no epidemic strains.